1. Field of the Invention
This invention relates generally to a device for tightening a through-bolt used in a stator core of a high voltage generator and, more particularly, to a device for automatically tightening a through-bolt that holds laminate plates together in a stator core of a high voltage generator while the generator is in service in response to detecting that the bolt has become loose.
2. Discussion of the Related Art
High voltage generators for generating electricity as a power source are well known in the art. A power plant may include gas turbine engines that each rotate a shaft by combusting fuel and air in a combustion chamber that expands across blades which rotate, and in turn causes the shaft to rotate. The output shaft of such an engine is coupled to an input shaft of a high voltage generator that is mounted to a rotor having a special configuration of coils. An electrical current provided in the rotor coils generates a magnetic flux around the coils, and as the rotor rotates, the magnetic flux interacts with windings in a stator core enclosing the rotor. The stator core windings include interconnected stator bars that have a special configuration to reduce eddy currents in the core, which would otherwise generate significant heat and possibly damage various generator components.
Stacked laminate plates in a stator core of this type are closely held together and compressed for proper operation of the generator to provide tight gas flow channels and the necessary sealing. During assembly of the generator, the laminate plates and stator bars are assembled in a vertical manner by sliding the components onto several circumferentially oriented bolts. For a typical generator, there may be sixty of these through-bolts, where the stator core may be about thirty feet long.
Once the stator core is in service and operating, it has an elevated temperature and is subject to vibrations and other stresses during normal generator operation. These forces and temperatures cause the various metal materials in the stator core to loosen so that, for example, the laminate plates are not as tightly packed and compressed as desired. Therefore, it is desirable to tighten the nuts on the bolts holding the stator core together to hold the plates in the desired state of compression. In order to tighten the bolts on the stator core, the generator needs to be taken out of service and disassembled, which is a complex and costly process. During maintenance of the generator, a technician will rotate the nuts using a torque wrench to ensure that the through-bolts are under the desired compression. However, because such a maintenance service on a generator is performed only periodically due to the costs involved, the generator may be operating without the desired compression in the stator core for extended periods of time. Also, the torque wrenches that are used for this purpose are not overly accurate in that the torque measurement provided by the wrench is subject to the friction of the nut on the threads.